I've been able to successfully change the zinc that's right after the water pump. But so far I've been unable to unfreeze the one that's at the back of the engine up on the cylinder head. I've tried WD-40 and a breaker bar, I've tried rapping it with a hammer to see if that would break it loose, but so far I'm not having any luck getting it to come loose. The fact that it's in a bad location for getting maximum leverage doesn't help either.

So... In the mean time while I struggle with this... How critical is that particular zinc in a Yanmar 2QM15 as long as the other one is new?

And if anyone has any suggestions for a freeing stubborn "square" bolt, I'm all ears.

I noticed in the service manual that the plate this zinc screws into (the same one that the temperature sensor screws into), comes off the engine with 4 small bolts. I may try to remove the entire plate. That way I can get it into a vise and get more leverage on the zinc bolt.

I'm assuming that there's nothing but water flowing behind this plate.

I had the same problem and removed the four smaller bolts with success. Put the plate in a vise added PB blaster, heat ect. and no cigar removing the larger bolt. Came to terms that this was how I was going to replace the zincs from now on. You'll have to make a new gasket, I made several for later use. Also suggest putting a little Never/Seize on those four bolts.
Bill,

How did you get the one off just after the water pump? Mine is stuck solid and the whole cylinder fitting wants to move instead of the zinc connection. Yanmar doesn't stock the cylinder fitting anymore, so replacing it may be difficult. Haven't tried PB Blaster yet, so that might work. The plates on the engine for the other zinc and temp sensor are still available if you need to replace it.

I had a problem with the zinc in the front of my engine as well. I ended up using vice-grips to unscrew the two short threaded tubes that the water lines attach to. Then I unscrewed the cylinder fitting from the block, put that in a BIG vice with a lot of help from a torch and a breaker bar I was able to free up the zinc.

Granted I got it out after I wasted 3 days trying to keep the cylinder fitting in place.

How did you get the one off just after the water pump? Mine is stuck solid and the whole cylinder fitting wants to move instead of the zinc connection. Yanmar doesn't stock the cylinder fitting anymore, so replacing it may be difficult. Haven't tried PB Blaster yet, so that might work. The plates on the engine for the other zinc and temp sensor are still available if you need to replace it.

The cylinder wanted to turn with mine too. But a pair of channel lock pliers keeping it from rotating did the trick for me.

I've been able to successfully change the zinc that's right after the water pump. But so far I've been unable to unfreeze the one that's at the back of the engine up on the cylinder head. I've tried WD-40 and a breaker bar, I've tried rapping it with a hammer to see if that would break it loose, but so far I'm not having any luck getting it to come loose. The fact that it's in a bad location for getting maximum leverage doesn't help either.

So... In the mean time while I struggle with this... How critical is that particular zinc in a Yanmar 2QM15 as long as the other one is new?

And if anyone has any suggestions for a freeing stubborn "square" bolt, I'm all ears.

I had the same trouble with mine.. YES,YES,YES it's very important both are replaced, if you allow the one you cannot get out to remain in place it will break down to a point where it will clog the water passages with material, then you'll be up for pulling the entire head off.

Mine don't have square head bolts so I was able to fit a socket on them. Took a lot to free the one on the back. I tried shock cooling it with stuff you buy in a pressure can from auto shops, that did not on it's own do much, tried "very" carefully heating it then cooling it it WD40.
Eventually I just had to brace myself against the hull in the quarter berth and put a foot on the spanner and push for all I was worth. It went with a "bang", since then I make sure I undo it every six months.

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